1. Field of the Invention
The present invention concerns a visibility measuring device of the type comprising a source directing an incident beam of modulated light onto a volume of ambient air and a receiver comprising a detector measuring the flow of modulated light diffused by the volume of air.
2. Description of the Prior Art
In a known device of this type (French utility patent No. 2214897), diffused light is measured only in the direction opposite that of the incident beam. It has been observed that the measurement of the light diffused by retrodiffusion does not possess a satisfactory correlation with the total diffusion coefficient of the volume of air. The information yielded by this type of known apparatus is thus marred by considerable error as a function of the nature of the aerosol. The applicants' studies show a non-compensatable systematic error (which is a function of the nature of the aerosol contained in the air) on the diffusion coefficient of between -50% and +100%.
Furthermore, the visibility measuring accuracy of this known apparatus further suffers since the measured accuracy based on the retrodiffusion principle is dependent on the physical condition of glass windows separating the beam source and the detector, e.g. stains on the glass windows adversely effect the accuracy of the visibility measurement.
In conclusion, while the process for measuring visibility by retrodiffusion is technically the simplest, it is also the least satisfactory in practice.